As a member of products in an automotive field, a household appliance field, or a building material field, surface-treated steel sheets to which rust prevention property is given are used. Among them, a hot-dip galvanized steel sheet is excellent in rust prevention property and is inexpensive, to thus be used heavily.
Generally, the hot-dip galvanized steel sheet is manufactured by the following method in general.
First, a thin steel sheet obtained by performing a hot-working hot rolling treatment, a cold-working hot rolling treatment, and a heat treatment on a slab is prepared as a base steel sheet (a base metal). Second, in a pretreatment step aiming at washing of the surface of the base steel sheet, degreasing and/or pickling are/is performed, or the base steel sheet is introduced into a preheating furnace without performing the pretreatment step, and thereby oil on the surface of the base steel sheet is burned to be removed. Third, the base steel sheet is heated to high temperature in a heating furnace (an annealing furnace), to thereby be subjected to recrystallization annealing. Fourth, the obtained base steel sheet is immersed in a hot-dip galvanizing bath, to thereby be subjected to a hot-dip galvanizing treatment. Incidentally, the base steel sheet is cooled down to a temperature suitable for plating prior to the immersion in a molten zinc bath.
Here, there will be explained a heat treatment atmosphere. The treatment atmosphere where the above-described recrystallization annealing is performed is set to an Fe reducing atmosphere. This makes it possible to suppress generation of oxides of Fe and to prevent or inhibit oxides of Fe from worsening plating wettability and plating adhesiveness in the subsequent hot-dip galvanizing treatment. Further, the treatment atmosphere of the above-described hot-dip galvanizing treatment is also set to an Fe reducing atmosphere similarly to the recrystallization annealing. Thereby, the hot-dip galvanized steel sheet can be manufactured continuously without being exposed to an oxidizing atmosphere such as the air.
Incidentally, the heating furnace used for performing recrystallization annealing in a continuous hot-dip galvanizing facility enabling the above-described continuous manufacture includes types such as a DFF (a direct firing type), a NOF (a non-oxidizing type), an all radiant tube type enabling the entire treatment atmosphere in the furnace to be changed into an Fe reducing atmosphere (a total reducing type), and combinations of them. At present, due to the point of easy operation, the point that roll pickup does not occur easily in the heating furnace, and the point that a high quality plated steel sheet can be manufactured at lower cost, a continuous hot-dip galvanizing facility using an all radiant tube type heating furnace has become widely used.
By the way, in recent years, in an automotive field in particular, among the hot-dip galvanized steel sheets, a hot-dip galvanized steel sheet in which elements such as Si and Mn are contained in a material of a base steel sheet and thereby the base steel sheet is increased in strength has been used increasingly. This is to satisfy a demand for achieving both an increase in strength of a member aiming at protection of passengers at the time of collision and a decrease in weight of a member aiming at improvement of fuel efficiency in the automotive filed.
However, Si and Mn are easily oxidizable elements as compared to Fe, so that there is caused a problem that Si and Mn contained in the base steel sheet are oxidized by heating for recrystallization annealing in the all radiant type heating furnace in spite of the treatment atmosphere being an Fe reducing atmosphere. Concretely, in the process of recrystallization annealing, Si and Mn existing on the surface of the base steel sheet are oxidized with a high probability, and in addition to this, thermally diffused Si and Mn are also oxidized in the vicinity of the surface of the base steel sheet, resulting in that oxides of Si and Mn are gradually concentrated in a surface layer of the steel sheet. Then, in the case when oxides of Si and Mn are concentrated in the surface layer of the base steel sheet, when the base steel sheet is immersed in a molten zinc bath in the subsequent hot-dip galvanizing treatment, the oxides of Si and the oxides of Mn exposed to the surface of the base steel sheet prevent the molten zinc and the base steel sheet from coming into contact with each other, to thus become a cause of worsening of plating wettability and become a cause of inhibition of plating adhesion to the base steel sheet.
As documents disclosing a technique for suppressing the concentration of oxides of Si and Mn described above, ones to be described below can be cited.
Patent Document 1 discloses that prior to a hot-dip galvanizing treatment, an oxidation treatment is performed on a base steel sheet in such a manner that a thickness of an oxide film to be formed on the surface becomes 400 to 10000 Å, and subsequently Fe is reduced in an in-furnace atmosphere containing hydrogen. Further, Patent Document 2 discloses that prior to a hot-dip galvanizing treatment, a surface portion of a base steel sheet is first oxidized, and subsequently an oxygen potential that determines a treatment atmosphere in a reducing furnace is adjusted, and thereby reduction of Fe and oxidation of Si inside the steel sheet (internal oxidation) are both controlled.
The techniques disclosed in these two documents are made by focusing on the recrystallization annealing process. Here, when a time period for reduction of Fe (reduction time period) is too long, removal of an oxide film of Fe can be performed, but concentration of oxides of Si in the surface layer of the base steel sheet is caused, and further when the reduction time period is too short, the oxide film of Fe remains on the surface portion of the base steel sheet. Then, realistically, when it is considered that the thickness of the oxide film formed on the surface of the base steel sheet by the oxidation treatment is non-uniform, there is caused a problem that the technique of adjusting the reduction time period described above alone is not sufficient for improving the plating adhesiveness. Further, when the thickness of the oxide film of Fe formed by the oxidation treatment is too thick, a matter in which the oxides are peeled off from the base steel sheet to attach to surfaces of rolls disposed in the furnace (roll pickup) is caused. In this case, there is also caused a problem that outlines of the oxides attached to the roll surfaces are transferred onto the surface of the following steel sheet and thereby quality is impaired (appearance flaws).
Further, Patent Documents 3, 4, and 5 each disclose a technique in which for the purpose of solving the above-described problems caused by oxidation of Fe and suppressing the aforementioned concentration of oxides of Si and Mn, prior to a hot-dip galvanizing treatment, during recrystallization annealing in an all radiant tube type heating furnace, an oxygen potential that determines a treatment atmosphere is increased up to the extent that Si and Mn are internally oxidized.
Similarly, Patent Documents 6, 7, 8, and 9 each disclose a technique of adjusting a treatment atmosphere used for a heating furnace.
However, in the techniques disclosed in Patent Documents 3 to 9, when the oxygen potential is increased too much, Si and Mn can be internally oxidized, but Fe is also oxidized, resulting in that the same problems as those described above are caused. On the other hand, even when the oxygen potential is increased up to the extent that Fe is not oxidized, internal oxidation of Si and Mn becomes insufficient, resulting in that oxides of Si and Mn are concentrated in the surface layer of the base steel sheet. Thus, either case causes a problem that the oxygen potential that determines a treatment atmosphere cannot be adjusted accurately. Therefore, by these techniques, a hot-dip galvanized steel sheet having uniform quality cannot be manufactured securely.
Further, as another example of the technique for suppressing concentration of oxides of Si and Mn, there can be cited a technique of employing a means of further increasing steps necessary for a general manufacturing method of hot-dip galvanizing described above. For example, Patent Document 10 discloses a technique in which annealing is performed two times prior to a hot-dip galvanizing treatment. Such a technique is regarded that when oxides of Si formed on the surface of a base steel sheet (surface concentrated substances) are pickled and removed after the first annealing is performed, formation of surface concentrated substances can be suppressed at the time of the second annealing. However, when the concentration of Si in the base steel sheet is high, the surface concentrated substances cannot be removed sufficiently by pickling, resulting in that there is caused a problem that plating wettability and plating adhesiveness cannot be improved sufficiently. Further, in order to remove the surface concentrated substances of Si, a facility for performing annealing two times and a facility for performing pickling are newly required, so that there is also caused a problem that facility cost is increased, and further production cost is also increased.
Further, as still another example of the technique for suppressing concentration of oxides of Si and Mn described above, there can be cited a technique in which prior to a plating step, Si and Mn are internally oxidized in a hot rolling step. For example, Patent Document 11 discloses a technique in which when manufacturing a hot-dip galvanized steel sheet in a continuous hot-dip galvanizing facility, an oxygen potential is adjusted in a hot rolling step, to thereby internally oxidize Si in a thin steel sheet (a base steel sheet). However, in such a technique, when rolling of the base steel sheet is performed in a cold rolling step following the hot rolling step, an internal oxide layer is also rolled simultaneously and a thickness dimension of the internal oxide layer is decreased, resulting in that in the subsequent recrystallization annealing process, oxides of Si are concentrated in a surface layer of the base steel sheet. Therefore, there is caused a problem that even by the technique, plating wettability and plating adhesiveness cannot be improved sufficiently. Further, in the technique, oxides of Fe are formed at the same time as Si is internally oxidized in the hot rolling step, but as described previously, there is also caused a problem that the quality of a steel sheet to be manufactured is impaired due to peeling of oxides of Fe.
Incidentally, the hot-dip galvanized steel sheet containing Si and Mn is not limited to the above-described problems (problems explained by using Patent Documents 1 to 11 as examples), and has a fundamental problem that workability (for example, ductility) of the base steel sheet is inferior to that of a hot-dip galvanized steel sheet not containing Si and Mn because the strength (hardness) of the base steel sheet is increased. Here, when the ductility of the base steel sheet is low, even if a contact between the hot-dip galvanizing layer and the base steel sheet is made well, for example, in the case when working (for example, press forming) is performed on the hot-dip galvanized steel sheet, a crack is caused in the base steel sheet itself or in an interface between the base steel sheet and the hot-dip galvanizing layer and thereby the hot-dip galvanizing layer becomes likely to be peeled off from the base steel sheet. That is, the hot-dip galvanized steel sheet containing Si and Mn is required to improve the plating adhesiveness more than the hot-dip galvanized steel sheet not containing Si and Mn is required.